I may be quoting the wrong law for what I am trying to explain, however the phonemenome is not dissimilar.

Using extension tubes or bellows to increase the distance from lens to film/sensor plane that allows closer focusing results in a fall off of light reaching the sensor, and exposure benifits by being increased for accurate results. If a macro lens is used its not a problem, unless for even closer focusing extension tubs/bellows is used.
I remember well doing the exercise at Wednesbury photo collage way back in 1966 to demonstrate the effect, film of course but principles don't change only technology.
Modern cameras with their sophisticated auto exposure systems automatically adjust for this so it goes largly unnoticed, but if a low level light source is used it could result in longer exposure than is ideal.
I will sometime today check my trusty photo encyclopaedia (dating back to 1966, might be collectors items now) to get the correct law.

Patrick

OK - yes, just like with teleconverters if you use an extension tube less of the light that is transmitted by the lens covers the frame so the brightness is reduced. In fact some macro lenses also lose light at very close focus even without extension tubes.